Cancellation of rf second-order intermodulation distortion

Cancellation of rf second-order intermodulation distortion

The disclosed invention relates to a transceiver system configured to cancel RF second-order intermodulation distortion (IMD2) within a differential reception path. In some embodiments, the transceiver system has one or more common-mode attenuation elements that attenuate common-mode interferer signals within the differential reception path. The common-mode attenuation elements detect a common-mode interferer from one or more nodes within a transceiver system front-end. One or more properties of the detected common-mode interferer are adjusted and then fed into the differential reception path, where the adjusted common-mode signal attenuates the common-mode interferers. In other embodiments, the differential reception path has a tunable amplifying stage. The tunable amplifying stage has first and second amplifier elements that are independently operated to intentionally introduce an operating parameter mismatch between the elements. The intentional mismatch may be tuned to account for different nonlinear responses of the amplifier elements to mitigate IMD2 within the differential reception path.Related Terms:AttenuateAttenuationIntermodulation DistortionTransceiverModulation

For example, FIG. 1a illustrates a wireless communication transceiver 100 comprising a transmitter section 102 and a receiver section 104. In order to reduce the hardware used by transceiver 100, a duplexer 106 may be configured to couple both transmitter section 102 and receiver section 104 to a common antenna 108. To achieve high data rates, transceiver 100 may be configured to operate in full-duplex mode, wherein both transmitter section 102 and receiver section 104 use antenna 108 at the same time. During full-duplex mode operation, transmitter section 102 typically uses one carrier frequency while receiver section 104 uses another carrier frequency.

Despite using different frequencies, intermodulation distortion may arise during operation of transceiver 100. Intermodulation distortion occurs when a modulated blocker passes a component with a nonlinear characteristic, forming a spurious signal (e.g., an additional signal at a frequency that are not at harmonic frequencies of a received signal, but are instead at a sum and difference of the original signal frequency) in a reception path that interferes with a received differential input signal.

Second-order intermodulation distortion is caused by multiplication of two interferer signals. FIG. 1b illustrates a frequency graph 110 showing an RF second-order intermodulation distortion (i.e., second order intermodulation distortion at RF frequencies) generated by interferer signals. As illustrated in graph 110, the frequency domain comprises a plurality of interferer signals at frequencies f1, f2, and f3. Although the frequencies of the interferer signals are not close to a received differential input signal frequency fR, the interferer signals may combine together to form spurious signals, 112 and 114, comprising products having a sum or difference of their frequencies (e.g., f1+f2, f3−f2). Spurious signals that land at an RF frequency occupied by received differential input signal frequency fR cause second-order intermodulation distortion that is detrimental to operation of the transceiver system. Once intermodulation distortion appears within the reception path, there is no way of distinguishing it from the desired signal and transceiver sensitivity is degraded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates a block diagram of a transceiver system with second-order intermodulation distortion in the reception path.

FIG. 1b is a graph showing the effect of second-order intermodulation distortion within the frequency domain.

FIG. 1c shows graphs illustrating the effect of common-mode interferers on a received differential input signal.

FIG. 2 illustrates a block diagram of an exemplary transceiver system configured to attenuate RF second-order intermodulation distortion within a reception path.

FIG. 3 illustrates a block diagram of an exemplary transceiver system comprising a tunable amplifying stage having one or more amplifier elements configured to introduce operating parameter mismatches between differential branches of a reception path.

FIG. 4 illustrates a block diagram of an exemplary transceiver system comprising one or more common-mode attenuation elements configured to attenuate common-mode interferer signals within a reception path.

FIGS. 5a-5b illustrate schematic diagrams of an exemplary tunable amplifying stage comprising one or more amplifier elements, as provided herein.

FIG. 9 is a flow diagram of an exemplary method for calibrating operating parameter mismatches between differential branches of a reception path.

FIG. 10 illustrates a flow diagram of an exemplary method for calibrating attenuation of common-mode interferer signals within a reception path.

FIG. 11 illustrates an example of a mobile communication device, such as a mobile handset, in accordance with the disclosure.

FIG. 12 illustrates an example of a wireless communication network in accordance with the disclosure.

DETAILED DESCRIPTION

The claimed subject matter is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. It may be evident, however, that the claimed subject matter may be practiced without these specific details.

It will be appreciated that “RF second-order intermodulation distortion” is second-order intermodulation distortion that is specifically located at an RF frequency (e.g., that is present in a receiver path before downconversion). This is in contrast to baseband (BB) second-order intermodulation noise, which is a distortion that is present at low, baseband frequencies (i.e., which deteriorates RX performance after downconversion). The disclosed apparatus and techniques disclosed herein relate to mitigation of RF second-order intermodulation noise.

RF intermodulation distortion typically occurs when a transmitted signal leaks into a reception path due to limited isolation provided by a duplexer. However, even modern duplexers that offer good isolation do so at the expense of a common-mode interferer component being fed to the reception path. Filters may be used to remove unwanted interferers from the reception path. However, such filtering is expensive to implement in products. Furthermore, if the filtering is not highly selective, interferers with considerable levels may still appear within the reception path. For example, as shown in graph 110, the frequency domain may be filtered, but still pass a frequency range Δf that includes a differential input signal and close by spurious signals.

The inventors have appreciated that there are two major mechanisms that contribute to RF second-order intermodulation distortion in differential transceiver systems. The first mechanism is mismatches between transistor devices in nonlinear RF amplifiers. In particular, there are operational mismatches between amplifier elements configured to amplify each of the two differential branches in a differential reception path. The operational mismatches lead to different nonlinear responses between the two differential branches and thus to RF second-order intermodulation distortion.

The second mechanism is the common-mode components of interferer signals. The common-mode components of interferer signals may cause problems when the interferer signals are processed alongside of a wanted differential input signal. For example, FIG. 1c illustrates the influence of a common-mode interferer signal on a total input signal for two differential branches in a reception path. A first differential branch 116 comprises an input signal having differential (DIFF) components, which are in phase with an interferer signal having common-mode (CM) components (i.e., a common-mode interferer signal). The resulting “total” signal has a first magnitude M1 equal to the sum of the differential and common-mode components. A second differential branch 118 comprises an input signal having differential (DIFF) components that are out of phase with a common-mode interferer signal. The resulting “total” signal has a second magnitude M2 that is different than magnitude M1, causing different second-order intermodulation distortion responses even when applied to a perfectly matched differential nonlinear amplifier in the reception path.

Accordingly, a transceiver system configured to cancel RF second-order intermodulation distortion is provided herein. The disclosed transceiver system is configured to cancel RF second-order intermodulation distortion arising from at least one of the two above identified mechanisms that contribute to RF second-order intermodulation distortion in differential transceiver systems.

In some embodiments, the transceiver system comprises a reception path having a tunable amplifying stage comprising first and second amplifier elements configured to respectively amplify differential components of a received differential input signal. The first and second amplifier elements are independently operated to intentionally introduce an operating parameter mismatch (e.g., a gain coefficient mismatch) between the first and second amplifier elements having a specific value. The intentional operating parameter mismatch may be tuned to account for different nonlinear responses of the two amplifier elements so as to mitigate RF second-order intermodulation distortion within the reception path.

In other embodiments, the transceiver system may additionally or alternatively comprise one or more common-mode attenuation elements. The common-mode attenuation elements are configured to attenuate common-mode interferer signals within the differential reception path. In particular, the common-mode attenuation elements are configured to detect a common-mode interferer signal from one or more nodes within a front end of the transceiver system. One or more properties of the detected common-mode interferer signals are adjusted and then the adjusted common-mode signal is fed back into the reception path, where the adjusted common-mode signal attenuates the common-mode interferer signals without substantially deteriorating a received differential input signal.

FIG. 2 illustrates an exemplary block diagram of a front end of a transceiver system 200 configured to mitigate RF second-order intermodulation distortion within a reception path.

Transceiver system 200 comprises a transmission path and a reception path. The transmission path is configured to provide a transmitted signal TX to an antenna 202 by way of a duplexer 204. The reception path is configured to receive an RF input signal from antenna 202. The RF input signal is provided to duplexer 204, which outputs a differential input signal along first and second differential branches, 206 and 208 respectively, of the reception path.

A nonlinear tunable amplifying stage 210 is configured to receive the differential input signal from duplexer 204. Tunable amplifying stage 210 comprises a first amplifier element 210a and a second amplifier element 210b. First and second amplifier elements, 210a and 210b, are configured to separately amplify components of the differential input signal before it is demodulated by a downstream mixer 216. For example, first amplifier element 210a is configured to amplify components of the differential input signal on the first differential branch, while second amplifier element 210b is configured to amplify components of the differential input signal on the second differential branch.

A control unit 212 is configured to provide control signals SCTRL to at least one of amplifier elements 210a and 210b. The control signals SCTRL independently control operating parameters of amplifier elements 210a or 210b. In some embodiments, control unit 212 is configured to provide a control signal to one of amplifier elements 210a and 210b. In some alternative embodiments, control unit 212 is configured to provide a first control signal to first amplifier element 210a and a different, second control signal to second amplifier element 210b.

By independently controlling amplifier elements 210a and 210b, which separately amplify different components of the differential input signal, an intentional mismatch in operating parameters of amplifier elements 210a and 210b may be introduced. The intentional operating parameter mismatch may be chosen to have a specific value (e.g., a relative gain coefficient mismatch factor) that accounts for mismatches between transistor devices within amplifier elements 210a and 210b. By accounting for mismatches between transistor devices within amplifier elements 210a and 210b, the nonlinear response of two differential branches may be reduced, thereby reducing RF second-order intermodulation noise. In some embodiments, the operating parameter mismatch may comprise a mismatch in gain (i.e., gain coefficients) of first and second amplifier elements 210a and 210b, for example.

Transceiver system 200 may alternatively or additionally comprise one or more common-mode attenuation elements 214. The common-mode attenuation elements 214 are configured to detect common-mode interferer signal(s) CMINT from one or more independent nodes in the front-end of transceiver system 200 (e.g., the transmit path or input of amplifying stage 206). One or more properties of the detected common-mode interferer signal(s) are adjusted (e.g., magnitudes, phases, group delays, etc.) to form an adjusted common-mode signal CMINT′ that is fed back into the reception path. Since the adjusted common-mode signal CMINT′ is based upon the detected common-mode interferer signal CMINT it may attenuate the common-mode interferer signal CMINT without affecting the differential input signal received from antenna 202.

In some embodiments, the adjusted common-mode signal CMINT′ may be fed back into the reception path at a downstream location. It will be appreciated that the term “downstream location” refers to a location that is downstream in relation to a detected interferer signal. For example, for an interferer signal that leaks from a transmission path to a reception path, any node within the reception path is downstream of nodes within the transmission path since the interferer signal flows form the transmission path to the reception path.

Control unit 212 may be further configured to provide control signals SCTRL to common-mode attenuation elements 214 to control adjustments to the one or more properties of the detected common-mode interferer signal CMINT. In some embodiments, one or more common-mode attenuation elements 214 are located within a feed-forward path. The feed-forward path may extend from a location upstream of tunable amplifying stage 210 to the output of tunable amplifying stage 210.

It will be appreciated that RF second-order intermodulation distortion within the reception path may be mitigated using one or both of the disclosed apparatus (e.g., tunable amplifying stage 210 or common-mode attenuation elements 214) depending on which RF second-order intermodulation generation mechanism dominates in a given transceiver design. For example, in some embodiments a transceiver system may comprise a tunable amplifying stage 210 but not common-mode attenuation elements 214. In other embodiments, a transceiver system may comprise one or more common-mode attenuation elements 214 but not a tunable amplifying stage 210.

For successful application of the proposed RF second-order intermodulation distortion reduction techniques, specific amounts/values of intentional mismatch and/or common-mode signal injection have to be determined. In some embodiments, a calibration element 216 may be configured to enable a calibration procedure that determines the specific amounts/values of intentional mismatch and/or common-mode signal injection. For example, calibration element 216 may be configured to provide test signals to one or more nodes in the front-end of transceiver system 200. Calibration element 216 then measures one or more parameters indicative of RF second-order intermodulation distortion caused by the test signal within the reception path, and adjusts settings of the common-mode attenuation elements 214 and/or tunable amplifying stage 210 to successfully reduce RF second-order intermodulation distortion.

In various embodiments, the calibration procedures may be done with a calibration element 216 comprising an external test source (i.e., as a “factory calibration”) or with a calibration element 216 comprising an internal test source (i.e., as a “field calibration”). Since the common-mode signal levels are determined to a large extent by passive external devices of the RF engine, using an external test source allows for the calibration task to be carried out based upon those devices.

For example, since RF second-order intermodulation distortion is largely introduced by the duplexer, the specifications of the duplexer may be used to properly determine how much the duplexer contributes to the generation of the unwanted common-mode signal, thereby effectively backing out the effect of the duplexer from the differential input signal. Furthermore, the use of an internal test source allows for the transceiver system to perform calibration dynamically during operation of the transceiver system, thereby allowing the system to account for changes that occur during operation. Exemplary calibration techniques are described in more detail below in exemplary methods 900 and 1000.

It will be appreciated that by reducing second-order intermodulation distortion that results from a transmitted signal leaking into the reception path, the disclosed apparatus and techniques allow for duplexers having a relatively low isolation to be used, thereby reducing the cost of the transceiver system. In some embodiments, the disclosed second-order intermodulation cancellation apparatus and techniques can sufficiently reduce second-order intermodulation distortion so as to provide a transceiver system that does not have a duplexer configured between the reception path and a transmitter path.

In particular, a duplexer 302 is connected to a differential reception path configured to conduct a differential input signal. When interferer signals, present in duplexer 302, are passed through a nonlinear amplifying stage 308 an RF second-order intermodulation distortion appears in the differential reception path.

The differential reception path has a first differential branch 304 and a second differential branch 306. First differential branch 304 is connected to a first input node Inp of a first amplifier element 308a and second differential branch 306 is connected to a second input node Inn of a second amplifier element 308b. First and second differential branches 304 and 306 are configured to respectively transmit a differential N-P complementary input signal from duplexer 302 to first and second amplifier elements 308a and 308b.

First amplifier element 308a is configured to receive first input signal components XP1 and XP2 at first input terminal InP. First amplifier element 308a operates upon first input signal components XP1 and XP2 to output a first order product, comprising a first order gain coefficient g1P multiplied by first input signal components XP1 or XP2 (e.g., G1PXP1 or G1PXP2), at a first output terminal OutP. First output terminal OutP also outputs a second order product comprising a second order gain coefficient g2P multiplied by a mixture of first input signal components XP1 and XP2 (e.g., G2PXP1XP2).

Second amplifier element 308b is configured to receive second input signal components XN1 and XN2 at second input terminal Inn. Second amplifier element 308b operates upon second input signal components XN1 and XN2 to output a first order product, comprising a first order gain coefficient g1N multiplied by second input signal components XN1 or XN2 (e.g., G1NXN1 or G1NXN2) at a second output terminal Outn. Second output terminal Outn also outputs a second order product comprising a second order gain coefficient g2N multiplied by a mixture of second input signal components XN1 and NX2 (e.g., G2NXN1XN2).

By expressing first and second input signal components, XP and XN, in terms of their common-mode components XCM and differential components Xdiff,

wherein the single-sided second-order gain coefficients, g2P and g2N, may be written as:

g2P=g2(1+Δg2) (4a)

g2N=g2(1−Δg2) (4b)

where Δg2 is a relative mismatch factor of the second-order gain coefficient, defined as Δg2=(g2P−g2N)/(g2P+g2N). Based upon these definitions, the second-order differential distortion signal YIMD2,DIFF may be written in terms of circuit parameters, as:

wherein XCM1 is the common-mode signal at a first frequency and XCM2 is the common-mode signal at a second frequency. The resulting RF second-order differential distortion signal YIMD2,DIFF has three components. The first two components are the cross products of the common-mode signals XCMx and the differential signals Xdiffx of the input interferer signals. The third term depends on mismatches between second-order gain coefficients Δg2 of the amplifying devices.

By intentionally mismatching gain coefficients of first and second amplifier elements 308a and 308b the differences in the nonlinear responses between first and second amplifier elements 308a and 308b may be removed. Removing differences in the nonlinear responses causes the relative mismatch factor of second-order gain coefficient Δg2 to go to zero, thereby reducing second-order differential distortion within the reception path by causing the third term of second-order differential distortion signal YIMD2,DIFF to disappear.

One of ordinary skill in the art will appreciate that the gain of a transistor device is a function of the transistor width to channel length ratio, such that increasing the channel width increases the gain of the device. Therefore, in some embodiments, the intentional gain coefficient mismatching of amplifier elements 308a and 308b may be performed by adjusting the effective widths of transistor devices within first or second amplifier elements 308a and 308b in an unequal manner. For example, the effective width of transistor devices may be changed in one of amplifier elements 308a and 308b without changing the effective width of transistor devices in the other one of amplifier elements 308a and 308b. It will be appreciated that the term “effective width” as applied herein refers to the cumulative width of transistor devices within an amplifier element. For example, since amplifiers typically comprise a plurality of transistor devices, the effective widths of transistor devices may be adjusted by adjusting the number of transistor devices that are turned on in an amplifier element.

In some embodiments, the existing circuitry used to change the effective width of transistor devices may be reused for intentionally mismatching gain coefficients of amplifier elements 308a and 308b. For example, in certain applications wherein device widths are modified to compensate for process and temperature variations, existing circuitry (e.g., switches, digital logic) may be reused to a large extent for intentionally mismatching of amplifier elements 308a and 308b.

FIG. 4 illustrates a block diagram of a front end of an exemplary transceiver circuit 400 configured to attenuate common-mode interferer signals. Transceiver circuit 400 comprises one or more common-mode attenuation elements configured to detect the common-mode of interferer signals, to adjust one or more parameters of the detected common-mode signals (e.g., amplitude, phase, delay, etc.), and then to feed the adjusted common-mode signal back into the reception path to cancel the common-mode interferer signals. By attenuating the common-mode interferer signals without adjusting the differential input signals, second order intermodulation noise is reduced without substantially deteriorating the differential input signals received by the antenna.

It will be appreciated that since the interferers may be separated from one another by a large frequency range (e.g., hundreds of MHz), transceiver circuit 400 may be configured to independently detect and adjust different common-mode interferer signals. In some embodiments, transceiver circuit 400 may comprise a first common-mode attenuation element 414 configured to attenuate common-mode interferers at a first frequency and a second common-mode attenuation element 422 configured to attenuate common-mode interferers at a second frequency.

For example, if transceiver circuit 400 is operated in full-duplex mode a first interferer signal 406 is generated from the transmitted signal in a transmitter section 402 leaking through duplexer 410 into a reception path of a receiver section 404. First interferer signal 406 may be attenuated by first common-mode attenuation element 414 connected between transmitter section 402 and receiver section 404. As illustrated in FIG. 4, first common-mode attenuation element 414 may comprise a conversion unit 416 configured to receive a single ended TX signal from a power amplifier 412. Conversion unit 416 converts the single ended signal TX signal to a common-mode signal and detects the common-mode of first interferer signal 406 therefrom. Conversion unit 416 is also configured to adjust properties of the detected common-mode signal. The adjusted common-mode signal is then injected into the reception path at a location downstream of a low noise amplifier (LNA) 418. The adjusted common-mode signal attenuates RF second-order intermodulation distortion within receiver section 404 (i.e., makes XCM1Xdiff2 approximately zero).

In various embodiments, conversion unit 416 may adjust properties including but not limited to the phase, magnitude, and/or signal delay of the detected common-mode of the first interferer signal. For example, in some embodiments, conversion unit 416 is configured to add a phase offset of 180° to the detected common-mode signal. The 180° phase offset causes the sign of the detected common-mode signal to change so that when the adjusted common-mode signal is reinserted into the reception path, the common-mode signal component (XCM1) is minimized (e.g., set to approximately zero). In some embodiments, a feedback receiver (FBR) input 420 may be used in power amplifier linearization systems as a reference node.

Transceiver circuit 400 may additionally or alternatively have a second common-mode attenuation element 422 comprising a feed-forward path configured to attenuate a second interferer signal 408. The feed-forward common-mode signal path extends from an input of low noise amplifier (LNA) 418 to an output of LNA 418. The feed-forward path comprises a common-mode detection element 424 and a common-mode adjustment element 426.

Second common-mode detection element 418 is configured to detect the common-mode of second interferer signal 408 in the reception path. Common-mode detection element 424 then outputs the detected common-mode signal to common-mode adjustment element 426. Common-mode adjustment element 426 is configured to make adjustments to properties of the detected common-mode signal independent from the common-mode signal detected in the transmitter section 402 (which may contain common-mode components at different frequencies). The adjusted properties may include but are not limited to the phase and/or magnitude of the detected common-mode signal of second interferer signal 408. The adjusted common-mode signal is then output into the reception path, attenuating RF second-order intermodulation distortion within receiver section 404 (i.e., making XCM2Xdiff1 approximately zero).

Accordingly, by utilizing two separate common-mode attenuation elements 414 and 422, the common-mode of multiple interferer signals (e.g., 406 and 408) may be effectively reduced. This reduces RF second-order differential distortion within the reception path by causing the first and second terms (XCM1Xdiff2 and XCM2Xdiff1) of the second order differential distortion signal YIMD2,DIFF disappear.

FIG. 5a illustrates a schematic diagram of a tunable differential amplifying stage 500 comprising two amplifier elements, 502 and 504, configured to operate as transconductors (e.g., having a transconductance gm). The effective width of one or more of amplifier elements 502 and 504 may be dynamically adjusted by one or more control signals to induce transistor device gain coefficient mismatches between amplifier elements 502 and 504. As shown in FIG. 5a, first amplifier element 502 is configured to receive a first control signal SCTRLp and second amplifier element 504 is configured to receive a second control signal SCTRLn, which is independent of the first control signal SCTRLp. Tunable differential amplifying stage 500 amplifies received differential input voltages Vinp and Vinn based upon control signals SCTRLpS and SCTRLn to generate differential output currents Ioutp and Ioutn.

FIG. 5b illustrates a circuit diagram 506 of an exemplary implementation of amplifier element 504 shown in FIG. 5a. Circuit diagram 506 comprises a plurality of amplifying transistor devices 508 and a plurality of switching transistor devices 510. Amplifying transistor devices 508 have gates connected to a node providing an input voltage Vinn, and drains connected to a node providing an output current Ioutn. Switching transistor devices 510 have a drain connected to a source of an associated amplifying transistor device, a source connected to ground, and a gate connected to a control word SCTRLn. The control word SCTRLx selectively activates switching transistor devices 510 to compensate for gain coefficient mismatches between amplifier elements 502 and 504. For example, when a switching transistor device (e.g., TSW—1, TSW—2, etc.) is turned on, its drain becomes connected to ground increasing the VGS and accordingly the current output from an associated amplifying transistor (since Id=K·(W/L)·(VGS−Vth)2). When the switching transistor is turned off, VGS of the amplifying transistor is decreased and the current output from the amplifying transistor is reduced.

Therefore, if the control word SCTRLn turns on transistor devices TSW—1 and TSW—2, the amplifier element 504 will have an effective width that results in a first gain coefficient and a first output current value. However, if control word SCTRLn only turns on transistor device TSW—1, the amplifier element 504 will have a smaller effective width that results in a second gain coefficient and a second output current value, which are respectively smaller than the first gain coefficient and the first output current value.

In some embodiments, the first and second control signals, SCTRLp and SCTRLn, may comprise tuning codes having a control-voltage, bit streaming, or control word, for example. In one embodiment, a control signal SCRTL comprising a digital control word having a plurality of k data bits is provided to a selection circuit 512. Based upon values of the plurality of k data bits in the received control word, the selection circuit 512 sends an activation voltage to selected switching transistor device gates, causing the selected switching transistor devices to turn on and thereby increase the effective width of the amplifier element 504. In some embodiments, the mean value of tuning codes provided to amplifier elements 502 and 504 may be determined by a transconductance alignment algorithm, wherein their difference is set in order to balance second-order transconductances.

Common-mode detection element 604 is configured to detect a common-mode voltage signal at the input of low-noise amplifier (LNA) 614 by using two matched resistors R1 and R2. The detected common-mode voltage signal is sent to paths 606 and 608, respectively comprising filter capacitors C1 and C2 which pass RF signals and that block DC signals, which provides the signals to adjustment element 610.

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